This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-277816, filed on Sep. 13, 2001, and No.2001-261182, filed on Aug. 30, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a bipolar transistor, a semiconductor light emitting device and a semiconductor device.
2. Related Background Art
GaAs, which is a III-V compound semiconductor, type transistors have various superior characteristics such as high operating frequency, low noise, high output, high gain, low operating voltage, high operating efficiency and low power consumption compared with Si transistors. Owing to these characteristics, GaAs type heterojunction bipolar transistors (hereinafter referred to as xe2x80x9cHBTxe2x80x9d) and GaAs type high electron mobility transistor (hereinafter referred to as xe2x80x9cHEMTxe2x80x9d) have been already put to practical use as mobile communication devices or the like.
Among these GaAs type transistors, the GaAs type HBT can be driven using a smaller number of power sources than the GaAs type HEMT and is therefore suitable for the miniaturization of system. Also, GaAs type HBT uses ballistic conduction of xe2x80x9chot electronsxe2x80x9d injected to a collector and hence has high speed operability. For this, the GaAs type HBT is expected largely to be a key device for supporting the communications using mobile tools such as portable telephones.
The portable telephone and the like generally require a micro-power amplifier ensuring high current gain by using an operation voltage as low as about 4.7 V or about 3.5 V. However, higher current gain is demanded of conventional GaAs type HBTs. Specifically, the same GaAs layer that is used for a base layer is conventionally used for an emitter layer, giving rise to the problem of reduced current gain because of the occurrence of inverse injection from the base layer to the emitter layer.
As a method of solving this problem, a bipolar transistor using an InGaP layer for the emitter layer is proposed in, for example, Japanese Patent Application Laid-Open No. 11-274167. This is an invention that InGaP having a larger bandgap than GaAs is used as the emitter layer to thereby decrease the aforementioned inverse injection. However, even the use of InGaP did not succeed in reducing much inverse injection because the bandgap was not large enough yet.
Also, in Japanese Patent Application Laid-Open No. 9-307100, a method using a wide gap semiconductor is proposed as a method for heightening dielectric resistance between a gate and a drain in a GaAs type HEMT. This is a method in which as wide bandgap semiconductor such as SiC or InAlGaN having a wider bandgap than the aforementioned InGaP is used for an supply layer in a GaAs type HEMT. However, the supply layer in HEMTs is a layer for supplying electrons to a high purity GaAs layer and therefore it is only required for the layer to have a film thickness of tense of nanometers. On the contrary, an n-type emitter layer in the GaAs type HBTs is one of layers constituting an npn junction in a transistor. So, the film thickness of the emitter layer must be about hundreds of nanometers to confine positive holes within a p-type base layer. For this, it is considered to be difficult to form a wide gap semiconductor as the emitter layer of the GaAs HBTs in the same method as in the case of the GaAs type HEMTs.
In light of this, the inventor of the present invention has attempted various experiments to raise the current gain of a GaAs type HBT by forming a heterojunction having a large difference in bandgap between an emitter layer and a base layer. As a result, the inventor has found independently that a HBT having high current gain can be obtained by using InGaN or InN for an emitter layer in a GaAs type HBT. Also, as a result of further experiments repeated by the inventor, it has been found that with regard to GaAs type semiconductor light emitting devices and the like, a high performance device can be obtained using such a method of forming a heterojunction having a large difference in bandgap.
The present invention has been conducted to solve the aforementioned problems and it is an object of the present invention to provide a higher performance semiconductor device by forming a heterojunction having a large difference in bandgap.
According to a first aspect of an embodiment of the present invention, there is provided a bipolar transistor comprising:
a substrate;
a collector layer with first conductive type formed on said substrate;
a base layer with second conductive type formed on said collector layer and made of a material selected from the group consisting of GaAs, InGaAs, AlGaAs, InAlGaP, InGaAsP, GaSb, GaAsSb, GaNAs, InGaNAs, SiGe, and HgCdTe; and
an emitter layer with first conductive type formed on said base layer and made of InpGa1xe2x88x92pN (O less than pxe2x89xa61), the emitter layer having a larger bandgap than said base layer.
According to another aspect of an embodiment of the present invention, there is provided a semiconductor light emitting device comprising:
A semiconductor light emitting device comprising:
a first conductive type clad layer;
an active layer formed on said first conductive type clad layer and made of InbAlcGa1xe2x88x92bxe2x88x92cAsdP1xe2x88x92d (0xe2x89xa6bxe2x89xa61, 0xe2x89xa6cxe2x89xa61, 0xe2x89xa6b+cxe2x89xa61 and 0xe2x89xa6dxe2x89xa61), the active layer emitting light by the injection of current; and
a second conductive type clad layer formed on said active layer and made of InrGa1xe2x88x92rN (0 less than rxe2x89xa61).
According to a further aspect of an embodiment of the present invention, there is provided a semiconductor device comprising:
a first semiconductor layer made of IntGa1xe2x88x92tN (0 less than txe2x89xa61) and a second semiconductor layer which forms heterojunction with the aforementioned first semiconductor layer, has higher electron affinity than the aforementioned first semiconductor and contains a material selected from the group consisting of GaAs, InGaAs, AlGaAs, InAlGaP, InGaAsP, GaSb, GaAsSb, GaNAs, InGaNAs, SiGe and HgCdTe.